Preference circuit for electronic musical instrument utilizing pulse amplitude discrimination and zero-crossing detector



Dec. 17, 1968 w M N H, J 3,417,188

PREFERENCE CIRCUIT FOR ELECTRONIC MUSICAL INSTRUMENT UTILIZING PULSEAMPLITUDE DISCRIMINATION AND ZERO-CROSSING DETECTOR Filed June 23, 19655| [FILTER FILTER IF: c1. 2

INVENTOR WALTER MUNCH, JR.

wmm BY MW/3T 1 m ATTORNEYS United States Patent Ohio Filed June 23,1965, Ser. No. 466,170 4 Claims. (Cl. 84--1.01)

The present invention relates generally to electronic musicalinstruments, and more particularly to preference circuitry forelectronic musical instruments employing a plurality of tone generatorswhich may individually control tone available in an output circuit, thepreference circuitry providing that simultaneous actuation of pluralkeys will produce only one tone in the output circuit.

Adjacent notes of a musical instrument, when played simultaneously, arenot harmonious. In the pedal division of an electronic organ,particularly, it is usual to provide a preference circuit to avoidsounding two notes simultaneously, when these are inadvertently calledforth by actuation of two pedal keys and usually the lower of any twonotes is preferred to the higher. Various types of preference circuitryhave been devised. Many of these have involved complex switchingarrangements and are therefore undesirable. Other arrangements haveinvolved networks of the ladder type. These networks involve simpleswitching arrangements, i.e. one single pole single throw switch pernote, but have the disadvantage that different tone signals pass to anoutput circuit via paths of different impedances, which modifies therelative levels of the several tones. In one such system the tonegenerator output circuits are required to be of low impedance, in orderto suppress non-preferred notes by acting as bypass circuit elements,but cross talk occurs because the bypass is not complete.

It is an object of the present invention to provide a novel preferencecircuit which operates with generator output impedances of relativelyhigh impedance, and provides output signals of the same amplitude at allfrequencies.

It is another object of the invention to provide a system for generatingzero crossings, responsive to the sum of plural tone signals ofdifferent amplitudes, which correspond in timing to the zero crossingsof only the tone signal of highest amplitude in the composite signal,and to produce tones in any output system for the system in response tothe generated zero crossings, whereby preference is'achieved withoutrequiring total suppression of non-preferred tones in the preferencenetwork itself.

It is a further object of the invention to provide response to that oneof two superposed, square waves, of unequal amplitude, which has thelarger amplitude.

A further object of the invention resides in providing a novel root toneselection system, i.e. a system in which only the lowest note of a chordplayed by a musician calls forth an audible response.

Briefly describing a preferred embodiment of the invention, a pluralityof square wave tone generators is connected in parallel between groundand separated points of a resistive string. The output impedances of thegenerators may be higher than or equal to the impedances betweenadjacent points of a string. Assuming the generators to be of decreasingfrequency progressively in proceeding from one end to the other of theresistive string, and that the latter end is the output end, the severalgenerators meet different impedances in proceeding to the output end, sothat the tone signal of lower frequency is always of greater amplitude.The sum of adjacent tone signals can then be shown to have adjacent zerocrossings at times established by the periods of the lower frequencytone signal only, with successive crossings of opposite slopes. The zerocrossings can be used to generate square waves, which can control outputtone generators, in the form of flip-flops. Zero crossings arecontrolled by the higher amplitude square waves, regardless of relativefrequencies, so that the system can be designed to prefer higherfrequencies or lower, as desired.

Reference is now made to the accompanying drawings, wherein:

FIGURE 1 is a schematic circuit diagram of a system according to theinvention; and

FIGURE 2 is an explanatory wave shape diagram, pertaining to FIGURE 1.

In the drawings, resistances 10, 11, 12, 13 are connected in series toform a resistance string. As many resistances as there are tonegenerators may form the string, and the resistances may be equal orunequal, but are shown as equal in the preferred embodiment. Thejunctions between resistance pairs, 10, 11; 11, 12; 12, 13 etc. aredenoted 20, 21, 22 and between these junctions and ground are connectedtone generators 25, 26, 27 each in series with a switch and a generatoroutput impedance, denoted respectively by reference numerals 25a, 25b;26a, 26b; 27a, 27b. Resistances 25b, 26b, 27b may be equal to each otherand to resistances 10, 11, 12, 13, if desired.

Resistance 10 proceeds to the base 30 of a transistor 31, having anemitter 32 connected to a negative voltage terminal 33. The latter isthe midpoint of a voltage divider extending from a negative voltagesource 34 to ground. The voltage may be 16 v. and the divider may becomposed of a 2.2K resistor 35 and a 1K resistor 36, so that junction 33is at approximately 5.2 v. Collector 37 of an NPN transistor 31 proceedsto ground via series resistances 38, 39, of 4.7K and 33K, respectively.The junction 40 of resistances 38, 39 is connected directly to the base41 of an PNP transistor 42, having an emitter 43 and a collector 44. Theemitter 43 is supplied with a -l v. supply and the collector 44 proceedsto a flip-flop 45, for control of the latter.

Transistor 42 operates over a small range of voltages above or belowwhich it saturates. Thereby it acts as a square wave clipper, whensupplied at its base with irregularly shaped Waves, and the square waveoutput is severely amplitude limited, for all amplitudes of theirregularly shaped waves, and has periods which are the same as timeintervals between the zero crossings of the irregularly shaped waves.

The generators 25, 26, 27 provide 8 v. peak-to-peak superposed on 4.5 V.DC Circuit values are then established so that approximately zerovoltage appears at point 40, in response to the 4.5 V. DC. alone. The 8v. peakto-peak square waves provided by generators then drive base 41 oftransistor 42 alternately more and less negative..However, a smallchange in signal at base 41 is adequate to drive the output oftransistor 42 between cut-off and l v., its emitter voltage.

Assume (FIGURE 2) that only E is present, i.e. the voltage derived, forexample, from generator 26, switch 26a alone being closed. Variations ofvoltage at base 41 occur, about zero base line 50, which gives rise tocontrol voltage 52 for flip-flop 45, driving flip-flop 45 into one stateat zero crossing 53, and into the opposite state at zero crossing 54. Asimilar set of events occurs in respect to generator 25, alone, having avoltage wave form E a lower frequency than is provided by generator 26.Here the zero base line is 55, and the changes of state occur at Zerocrossings 56, 57.

If switches 25a and 26a are closed together, signal from tone generator26, i.e. E is attenuated more than is signal from tone generator 25,i.e. B Assume that for 1 E we have /2 E merely to simplify expositon.This will be the situation for any two adjacent tone generators, ifgenerator output impedance is equal to impedance between adjacentconnection points of the string. The sum of E /2 E is then illustratedas the lowermost wave form of FIGURE 2, and it will be observed thatboth the rising and falling zero crossing, at points 60, 61 arecontrolled by the zero crossings of the E wave form alone, and are welldefined, i.e. occur between values /2 E and /2 E,,. It follows that thecontrol signal applied to flipflop 45 is at the frequency of E the lowerfrequency of the two active generators. However, were higher frequencysignals of greater amplitudes, preference would be in favor of thehigher frequency tones, rather than the lower.

While a simple ladder circuit design according to the present system,permits equality of all ladder resistances, such equality is notrequired and may not be optimum. Resistances b, 26b, 2712 may be locatedon either side of switches 25a, 26a, 27a, and in either positionrepresent generator output impedances. As one moves to the left in theladder configuration of FIGURE 1, the ratio of amplitudes, as betweenE014) and E remains constant, although the total current supplied totransistor base 30 decreases the total current being adequate out tothirteen tone generators, the usual condition in organs, which havethirteen pedal keys.

The flip-flop may drive a further flip-flop 50, to form a frequencydivider stage, i.e. the output of flipflop may be an octave below thatof flip-flop 45. By the same token, flip-flop 45 may itself be afrequency divider. In such case, if keys 25a, 26a, 27a are key operatedswitches of an accompaniment manual, flip-flop 45 may form an output ofa root tone system, i.e. a system which plays a single tone one octavebelow the lowest note of a' chord played on the accompaniment manual,however complex that chord may be.

The flip-flops 45, 50 may each be in cascade with a tone color filter,as 51, 52, selectively connectible to an output system, such as a loudspeaker 53, by means of tone color filter selection switches 54, 55.

Essentially, the present invention provides that, of an array of tonegenerators of an electronic organ having progressively decreasingfrequencies, the lowermost frequencies shall have progressively thehigher amplitudes. Tones called forth by key switches are additivelycombined. It can then be shown that the combined tones, although forminga complex wave shape, have clear cut zero crossings at the frequency ofthe tone of lowermost frequency. The zero crossings are utilized togenerate or control output tones for the organ. The ladder network ofFIGURE 1 then represents only one system for generating the requiredarray of tones. The principle of the invention permits the highestamplitude square wave of a plurality of such waves to be selected,whether or not of lowest frequency, and the ladder network utilizedrepresents merely a convenient device for assuring the requiredamplitude differentials where the tone generators have the same outputvoltages.

While I have described and illustrated one specific embodiment of myinvention, it will be clear that variation of the details ofconstruction which are specifically illustrated and described may beresorted to without departing from the true spirit and scope of theinvention as defined in the appended claims.

What I claim is: 1. In a preference system for square waves of differentfrequencies,

.a plurality of sources of square wave tones of progressively decreasingfrequencies and correspondingly modified amplitudes,

means for at will selecting a combination of said sources andtransferring the corresponding square wave tones to a common circuitelement, and

means responsive only to zero crossings of signal in said common circuitelement for generating a musical tone, whereby the generated musicaltones have a frequency locked to the frequency of only one preferredfrequency source of the selected combination of said sources.

2. In a reference network,

a plurality of tone signal generators of progressively decreasingfrequencies, each of said tone signal generators having a relativelyhigh output impedance,

an impedance string having plural taps distributed therealong,

means for selectively connecting said tone generators one for one tosaid taps,

an output'system connected to one end of said resistance string,

means for detecting zero crossings of signals in said output system, and

means responsive to said zero crossings for generating wave forms lockedin frequency to the frequency of the lowermost signal generatorselectively connected to said taps by said means for selectivelyconnecting 3. In a preference system,

a plurality of tone signal generators of progressively decreasingfrequencies,

means for progressively modifying the amplitudes of the tone signals asa function of said frequencies,

a control device,

means for at will selectively connecting said tone signal generators tosaid control device,

said control device including means for generating rectangular waveshaving transitions occurring in response only to zero crossings of tonesignal at said output system, whereby only the frequency tone signalhaving greater amplitude of plural tone signals at said control devicecontrols said transitions.

4. In a root tone generating system, I

a plurality of musical tone generators of progressively decreasing tonefrequencies,

switch means for at will selecting combinations of said tonefrequencies, and

means for generating a single tone octavely related to the lowermostfrequency of the selected tone frequencies to the exclusion of theremaining selected tone frequencies, said last means comprising meansfor relatively decreasing the amplitudes of the musical tones as afunction of increasing frequency to form leveled tone signals, means foradditively combining said leveled tone signals into a composite waveform, said composite wave form having zero crossings corresponding intime with the zerp crossings of only that one of said leveled tonesignals having the lowest frequency, and means responsive to the zerocrossings of said composite wave form for generating said root tone.

No references cited.

ARTHUR GAUSS, Primary Examiner.

R. H. PLOTKIN, Assistant Examiner.

U.S. Cl. X.R.

4. IN A ROOT TONE GENERATING SYSTEM, A PLURALITY OF MUSICAL TONEGENERATORS OF PROGRESSIVELY DECREASING TONE FREQUENCIES, SWITCH MEANSFOR AT WILL SELECTING COMBINATIONS OF SAID TONE FREQUENCIES, AND MEANSFOR GENERATING A SINGLE TONE OCTAVELY RELATED TO THE LOWERMOST FREQUENCYOF THE SELECTED TONE FREQUENCIES TO THE EXCLUSION OF THE REMAININGSELECTED TONE FREQUENCIES, SAID LAST MEANS COMPRISING MEANS FORRELATIVELY DECREASING THE AMPLITUDES OF THE MUSICAL TONES AS A FUNCTIONOF INCREASING FREQUENCY TO FORM LEVELED TONE SIGNALS, MEANS FORADDITIVELY COMBINING SAID LEVELED TONE SIGNALS INTO A COMPOSITE WAVEFORM, SAID COMPOSITE WAVE FORM HAVING ZERO CROSSINGS CORRESPONDING INTIME WITH THE ZERO CROSSINGS OF ONLY THAT ONE OF SAID LEVELED TONESIGNALS HAVING THE LOWEST FREQUENCY, AND MEANS RESPONSIVE TO THE ZEROCROSSING OF SAID COMPOSITE WAVE FORM FOR GENERATING SAID ROOT TONE.